Density Profile of a Cool Core of Galaxy Clusters

Abstract

The density profile of a cool core of intracluster gas is investigated for a cluster of galaxies that is initially in the virial equilibrium state, and then undergoes radiative cooling. The initial gas profile is derived under the assumption that the gas is hydrostatic within the dark-matter potential presented by the NFW or King model, and has a polytropic profile. The contribution of masses of gas and galaxies to the potential in the calculation is ignored compared to the dark matter. The temperature and density profiles of gas in its quasi-hydrostatic cooling phase, which is expected to last for ~Gyr, is then calculated for different initial gas profiles. It is found that in the quasi-hydrostatic cooling phase, while the temperature decreases to about 1/3, the density increases by a factor of 4-6 at the cluster center in comparison with their initial polytropic values, though the profiles over the core depend on the dark-matter potential. Hence, the core radius in the quasi-hydrostatic cooling gas appears to be smaller than that in the initial polytropic gas. We compared the density profile of the cool core with observations to find that, while the initial density is around the upper bounds of large-core (>100 kpc) clusters, most likely relaxed, but the cooling is not yet significant, the central density under quasi-hydrostatic cooling falls between the mid- and high-values of small (<100 kpc)- or cool-core clusters. It is also found for the quasi-hydrostatic cooling gas that the entropy profile roughly agrees with the best-fit model to the ACCEPT sample with a low central entropy; also, the pressure gradient in the inner core is close to that of the REXCESS sample. X-ray surface brightness calculated for the quasi-hydrostatic cooling gas is well represented by the conventional double beta-model, giving a physical basis for applying the double beta-model to cool-core clusters.